Devices for shaking the control column or "stick" which controls fixed-wing aircraft are well known and provide an unmistakable warning of an impending stall. Such devices are typically mounted on the aircraft's control column or "stick" and provide a high amplitude, low frequency vibration to signal the pilot of an impending stall. Such signals provide an unmistakable warning to the pilot, which cannot be confused with other warnings and signals typically present in an aircraft cockpit. Control column shakers of various designs are manufactured and sold by Safe Flight Instrument Corporation of White Plains, N.Y., the assignee of the present invention.
The use of a control column shaker in rotary wing aircraft is also known, as evidenced by U.S. Pat. No. 4,115,755 to Cotton. As disclosed therein, an alarm, such as a shaking of the collective pitch control lever of a helicopter, is used as an indication that the actual acceleration exceeds the permissible acceleration. As disclosed by this reference, the structural envelope of the helicopter, as defined by weight, air speed, rotor rpm, air density and collective stick position, is used as an indication of permissible loading of the helicopter rotor. Cotton also teaches the use of a gross measure of the aircraft weight, density of the air, air speed, collective pitch and rotor speed of the aircraft as indices of a table look-up to determine the maximum permissible aircraft acceleration under such flight conditions, which is then compared against the actual aircraft acceleration to determine when the aircraft is maneuvered close to a loading which exceeds the structural envelope.
Controls for a single rotor helicopter typically include a cyclic control stick which movement controls the movement of the helicopter about the roll axis and the pitch axis, a collective control stick which controls the vertical movement of the helicopter by collectively adjusting the pitches of the rotor blades, and foot pedals which control the orientation of the helicopter about the yaw axis. Typically, such yaw control involves a tail rotor located near the end of the tail boom of the helicopter and rotating in a plane generally perpendicular to the plane of rotation of the main rotor. Movement of the fuselage of the helicopter about the yaw axis is achieved by pushing on one of the left and right foot pedals, which movement typically adjusts the pitch of the tail rotor thereby increasing, or decreasing the force on the tail boom which counteracts the rotational force exerted on the fuselage of the helicopter by the power plant during rotation of the main rotor blade.
While it is known to incorporate a force mechanism through the tail boom of the helicopter to direct a jet of air or other gas laterally through the end of the tail boom or over the exterior of the tail boom, thereby eliminating the need for a tail rotor, the magnitude of such a lateral force is controlled by the foot pedals so as to control the yaw orientation of the helicopter fuselage.
Given the ever increasing torque capabilities of modern helicopter power plants, circumstances may arise, especially under rapid helicopter ascent, in which the force exerted by the tail rotor, or other yaw control force mechanism, is incapable of counteracting the counter-rotational force exerted on the fuselage by the aircraft power plant. Such circumstances, quite obviously, are undesirable and may result in the loss of control of the helicopter by the pilot.
It is believed that there is a significant need for a warning system which will alert the pilot as to the impending maximum force exerted by the tail rotor, or other force mechanism, in a direction to counteract the rotational forces imposed on the helicopter fuselage by the power plant. Given the warning of an approach of such maximum force, it may then be possible for the pilot to adjust the other helicopter controls to avoid any undesired movement of the helicopter fuselage about the yaw axis due to insufficient rotor force. Such a warning system should provide an unmistakable warning to the pilot and should not be confused with other aircraft warning signals.